1. Field of the Invention
This invention relates generally to dehumidifiers, and more particularly to a dehumidifier that uses a membrane cartridge for dehumidifying air and other gases.
2. Description of the Related Art
Dehumidifying systems are used in a variety of different applications. For example, air dehumidifying systems are used in applications ranging from dehumidification of offices for maintaining comfortable working areas during summer months to providing dry air for dental tools. Different applications often require different levels of humidity A humidity level of about 40% to 60% is comfortable in homes or offices, while a humidity level of less than 10% is desirable in certain laboratory situations. Even lower humidity levels are often desirable in communications systems.
Commonly used signal transmission media in communications systems are waveguide, coaxial cable, multiwire telephone cables, and optical fiber cables. Changing environmental conditions can affect the overall performance of a system using any of these media. For example, when the temperature of air inside a waveguide or other transmission medium falls below its dew point, condensation occurs inside the transmission line. Condensation lowers the efficiency of waveguide and coaxial cable systems partially because the dielectric constant of water is greater than the dielectric constant of air, and partially because the condensation alters the impedance of the waveguide or coaxial cable and may produce signal variation or loss. In multiwire cables, condensation can lower the insulation resistance and introduce undesirable leakage paths.
To prevent the accumulation of moisture in such systems, the transmission line is normally sealed and pressurized to prevent the ingress of moisture through any small gaps. To prevent condensation within the system, the pressurization is effected with dry air from a dehumidifier or dehydrator. A compressor or pump typically supplies the pressurized air, and the dehumidifying apparatus removes moisture from the pressurized air before it is injected into the system. The low moisture content of the air lowers the dew point so that condensation does not take place except at very low temperatures. Moreover, due to the small amount of moisture present in the injected air, only a small amount of condensate can form even at unusually low temperatures.
One type of dehumidifier is commonly referred to as a desiccant dryer. Typically, a desiccant container holds a hydroscopic agent, such as silica gel, calcium oxide or sulfuric acid, and a gas such as air is pumped through the container. Since the desiccant has a great affinity for water, moisture within the air is attracted to the desiccant. Therefore, gas leaving the container contains little moisture. However, the hydroscopic agent eventually becomes saturated or ineffective and requires regeneration or replacement. Due to this shortcoming, continual operation is impossible where only one desiccant container is used. Moreover, many of the hydroscopic agents, such as sulfuric acid, are hazardous and expensive.
Another type of dehumidifier is commonly referred to as a refrigerant dryer Refrigerant dryers cool a compressed gas below the ambient temperature so that moisture in the compressed gas condenses on refrigerated coils. In contrast to desiccant dryers, refrigerant dryers have the advantage of being able to continually remove moisture from the gas. However, refrigerant dryers require large quantities of energy, and dehumidification of gases to low humidity levels is difficult.
Another type of dehumidifier, commonly referred to as an automatic pressure-sensing regenerative dryer, uses two cylindrical towers containing molecular sieve material. The two towers are alternately cycled so that while one chamber is drying the gas passing through it, the other chamber is being purged of accumulated moisture These two towers vent to the atmosphere through a solenoid valve activated by a timing motor. This type of dryer is inherently noisy and requires a considerable number of electro-mechanical parts, which reduces the reliability of the system.
It has also been known that gases can be dried by passing them through a membrane cartridge containing multiple membranes through which moisture, but not the gas being dried, can permeate and escape to the atmosphere or a collection system. The membranes in such cartridges, which are commercially available, are typically in the form of hollow fibers so that a gas may be passed through the interiors of the fibers while moisture removed from the gas is collected from the exteriors of the fibers.
A problem with these membrane cartridges, however, is that the membranes absorb moisture and lose their effectiveness when the compressor or other source of pressurized air is turned off. In the case of pressurized waveguide systems, for example, the compressor is normally off for much longer periods than it is on, because it is supplying pressurized air to a sealed system. When a membrane cartridge is used in such a system, the membranes must be dried to restore its effectiveness each time the compressor is turned on; this restoration time is sufficiently long, typically 20 minutes or more, that it seriously degrades the efficiency of the pressurization-dehumidification system.
The present invention is directed to overcoming one or more of the problems as set forth above.